Electrical negative-glow discharge display devices

ABSTRACT

An electrical display device employing a two-dimensional array of glow discharge cells formed by an insulating plate having a plurality of apertures therein defining a plurality of hollow cathodes. Each of these hollow cathodes is formed by a cathode pin on which a layer of emissive material is provided in each of the apertures spaced from one surface of the plate. The walls of each aperture are also covered with material sputtered thereon and thus forming a cathode recess, or hollow cathode in the plate. An anode common to all the cathodes and spaced from the edge of each recess to prevent cathode-anode tracking of sputtered cathode material is provided. The cathode recesses are viewable through this anode which may be in the form of a grid or transparent plate spaced from the cathode recesses. Each of the cathodes is connected to a common negative electrode through an element of photoconductive material and allows passage of input radiation to each of those elements to provide controlled feedback to each of the cells.

United States Patent [72] Inventor Pieter Schagen Surrey, England [2]]Appl. No. 699,267 [22] Filed Jan. 19, 1968 [45] Patented Jan. 5, 1971[73] Assignee U.S. Philips Corporation New York, N.Y. a corporation ofDelaware. by mesne assignments [32] Priority Oct. 26, 1967 [3 3] GreatBritain [31 No. 2,702/67 [54] ELECTRICAL NEGATIVE-GLOW DISCHARGE DISPLAYDEVICES 9 Claims, 19 Drawing Figs.

[52] U.S. Cl. 250/213, 315/169 [51] lnt.Cl 11011 17/00 [50]FieldofSearch 250/213; 315/ 169 [56] References Cited UNITED STATESPATENTS 2,837,661 6/1958 Orthuber et al. 250/213 Metal pnode AiTa)2,972,803 2/1961 Kouryetal. 3,334,269 8/1967 LHeureux ABSTRACT: Anelectrical display device employing a twodimensional array of glowdischarge cells formed by an insulating plate having a plurality ofapertures therein defining a plurality of hollow cathodes. Each of thesehollow cathodes is formed by a cathode pin on which a layer of emissivematerial is provided in each of the apertures spaced from one surface ofthe plate. The walls of each aperture are also covered with materialsputtered thereon and thus forming a cathode recess, or hollow cathodein the plate. An anode common to all the cathodes and spaced from theedge of each recess to prevent cathode-anode tracking of sputteredcathode material is provided. The cathode recesses are viewable throughthis anode which may be in the form 'of a grid or transparent platespaced from the cathode recesses. Each of the cathodes is connected to acommon negative electrode through an element of photoconductive materialand allows passage of input radiation to each of those elements toprovide controlled feedback to each of the cells.

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PIETER SCHAGEN ELECTRICAL NEGATIVE- GLOW DISCHARGE DISPLAY DEVICES Thisinvention relates to electrical discharge display devices. The inventionrelates to some extent to relatively simple devices for displayingsimple patterns such as diagrams, numerals, words and the like which donot require half-tone I capabilities. The invention also relates todevices for displaying more complex images such as radar displays which,however, may also not require half-tones. However, the invention relatesprincipally to devices capable of producing televisiontype or X-ray typeimages containing a range of half-tones and, for this reason, thefollowing description is given mainly in terms of devices of this lattertype.

During the past few years an increasing amount of effort has beendevoted to flat display systems of one kind or another many of thembeing of the solid-state type. Probably the most v serious problem withthe all solid-state approach to this kind of display is the picturebrightness which can be achieved. This problem has two aspects:

a. A sequential address system makes it virtually impossible to obtain asatisfactory brightness from individual picture points unless somestorage mechanism is employed which enables each point to emit light,even when the exciting signal is no longer present. Preferably emissionshould continue until the next "scan" of the element occurs. This makesthe associated circuitry and the panel construction very complicated.

b. If the light-emitting phenomenon employed is that ofelectroluminescence, the present state of the art in this field does notpermit a brightness in excess of a few tens of ft.-l. even for constantemission. This is not sufficient for daylight viewing purposes asrequired in the majority of the possible applications.

Forthis reason it has been proposed to employ a twodimensional array ofcold-cathode glow-discharge cells as the light sources for theindividual picture elements. An early example of such'an arrangement isdescribed in US. Pat. No. 2,991,394, issued July 4, 1961 and illustratedin FIGS. -11 thereof.

Amore recent example is the Lear Siegler device reported in an articlein Electronic News of July 26, 1965. In this proposal the individualcells consist of positive-column discharge spaces arranged in the formof a two-dimensional matrix of small apertures in a plate of insulatingmaterial placed between two electrode systems of parallel wires orsemitransparent conductive strips in a crossbar arrangement. Any elementcan be made to emit light by passing a gas discharge through a hole inthe insulating matrix at the crossover point of the correspondingcathode strip and anode strip.

Although with this particular arrangement it is possible to obtain ahigh picture brightness, this cannot be realized in practice unless itis used in a bistable on-off mode, because each element can only beexcited to give its correct light output at the instant during which itis scanned, or at most during a full line period. The latter can(although not described in the aforesaid article) be achieved if theincoming picture signal for each line is written into a line storageelement during one line scan time, and displayed simultaneously as acomplete line during the entire scan time of the picture signal for thenext line, which is then being written into a second line store.

Another difficulty with gas discharge devices of this Lear type is thesputtering which takes place from the cathode, due to the bombardmentwith positive ions. This can severely limit the life of the device andcan occur in various ways, for example:

a. Since the cathode strips are thin, it is possible for a substantialpart of the cathode material of a cell to be removed from its stripthereby affecting its performance. Serious differences in performancemay thus arise between one cell and another clue to random removal ofcathode material.

b. The sputtered material from a cathode may in time set up partiallyconductive tracks between adjoining cathodes thus allowing the glowdischarges to spread and severely disturbing the operation of adjacentcells.

c. The transparent anode strips may be rendered opaque by deposition ofsputtered material thus impeding viewing of the display through theanode plate.

d. If the anode and cathode plates are in direct physical contact witheach side of the matrix, the sputtering may in time set up partiallyconductive tracks from cathodes to anodes thus severely disturbing theoperation of'individual cells.

Similar sputtering problems occur also in individual cells, and recentwork on single and multicell devices by Dr, R. F. Hall has indicatedthat at least some of these problems can be successfully overcome by theuse of an array of recessed cathode glow-discharge cells wherein eachcathode is recessed in a slab of insulating material. As described inUS. Pat. No. 3,465,194 issued Sept. 2, 1969 one such example is shown inFIGS. 1 and 2 of the accompanying drawings, FIG. 1 showing a corner ofthe cathode array separate from the envelope or cover (containing asemitransparent anode layer) and FIG. 2 showing the two parts assembledtogether.

It has been found that the cathode recesses can effectively restrict thesputtering action even after the walls of the recesses have beenrendered conductive by sputtered cathode material.

With such an arrangement a very high brightness can be obtained, morethan adequate for normal display purposes. In addition, it has beenshown that the arrangement can reduce the sputtering to such an extentthat lifetimes in excess of several thousands of hours are possiblewithout any apparent deterioration in the characteristics of thedischarge.

It has also been shown that the characteristic of each individualrecessed cathode is in practice substantially independent of itsneighbors, is substantially linearly related to the current fed into it,and allows a contrast ratio of at least a few hundred to one. Thus anarrangement of the type shown in FIG. 2 can provide sufficientbrightness if many or all of the cells are activated simultaneously toproduce an image, but with a scanned or dot-sequential mode of operationthe brightness problem remains.

It is an object of the invention to provide an improved recessed-cathodeglow-discharge display panel suitable for use by itself as an imageintensifier and suitable also for use in combination with driving meansadapted to activate individual cells or groups of cells in apredetermined sequence.

The invention provides an electrical display panel device comprising atwo-dimensional array of glow-discharge cells, an individual recessedcathode (as. herein defined) for each cell, an element ofphotoconductive material coupled externally to each of said cathodes soas to be in series therewith, a common anode for all the cells throughwhich anode an image composed of glow discharges can be viewed, and acommon negative supply electrode for all the cells which electrode isconnected to all the cathodes through said elements of photoconductivematerial and allows passage of input radiation to each of said elements.

The common anode may be a continuous layer which is transparent (to anyrequired degree) to the output radiation provided by the glow dischargesof the cells. Alternatively, the anode may be formed as a mesh of opaqueconductors between which the glow discharges can be viewed.

In a similar manner, the common negative supply electrode may be acontinuous layer which is substantially transparent to the type ofradiation which is intended to be used as the input to the panel. Thus,in the particular case of an X-ray image intensifier application, thelayer may for example be opaque to visible radiation thoughsemitransparent to the type of X-rays for which the device is designed.As an alternative to a continuous layer, the common negative electrodemay be formed as a mesh of opaque conductors between which the inputradiation can be admitted to the elements of photoconductive material.

In a somewhat analogous way the device may employ photoconductiveelements formed as a single continuous layer arranged to be common toall the cells in such manner that the photoconductive action is in thedirection of thickness of said layer.

Alternatively, the device may employ a separate photoconductive elementfor each cell. In this case the device may be so arranged that thephotoconductive action of each element is substantially in directionsparallel to the image of the display device.

In any event, input radiation will reach the photoconductive elements soas to actuate them (either simultaneously or sequentially) withdiffering intensities so as to set up an array of glow dischargesproviding the desired display. In this process each element ofphotoconductive material can perform two functions, namely (a) to passto its cell a discharge current related to the intensity of the inputradiation at the point, and (b) to act (in the sequential case) as astorage element so as to increase the duration of the glow discharge andhence the brightness of the display.

For the purposes of this specification a recessed cathode is, broadly,one which provides an exposed emissive surface surrounded by a wallwhich may be of insulating material or of conductive material. In thelatter case the said material may be provided by construction or bysputtering and may or may not be in electrical contact with the emissivesurface while being insulated from the walls of neighboring recesses. Inthe case of cathode-glow operation, the recess (and other parameters ofthe device) can be designed in known manner so that the glow iscontained within it, and this can be done regardless of whether thesurrounding wall is conductive or not. Hence the volume of the cathodeglow can be held substantially constant in spite of progressivesputtering of the walls of the recess.

The drawing will be described with reference to the accompanying drawingin which:

FIGS. 1 and 2 show a prior art display device;

FIGS. 3 and 4 are elevational views of two embodiments of a displaydevice according to the invention;

FIGS. 5A and 5B are an elevational and plan view respectively of anotherembodiment; and

FIGS. 6A to 6M show various other embodiments of a device according tothe invention.

As a simple preliminary example, the arrangement may be similar to thatof FIGS. 1-2 with a layer P of sintered CdSe or other photoconductorapplied to the outer side of the glass slab G and metal cathode plugs Kpas shown in FIG. 3 of the accompanying drawings, and a continuouselectrode E being applied to the exposed side of the photoconductivelayer P.

The gas space may be subdivided into individual cell spaces, but this isnot desirable in itself, particularly when using glow discharges whichare adequately located by, and contained in, individual recesses.Notional (i.e. nonairtight) subdivision of the gas space into individualcell spaces by a cellular matrix of insulating material may beconvenient in that it permits a relatively cheap and simple sandwichconstruction, but even then it is desirable to provide a gap to separatethe edge or rim of each recess from the anode so as to meet theaforesaid problem of sputtered conductive tracks from the cathodes tothe anode. These two requirements can both be met by appropriatesandwich" arrangements as will be explained.

In addition to cathode-glow operation, a device according to theinvention can be designed to provide also a positive column glow as willalso be explained. One arrangement (which will be described) employs ananode recess corlumination. Secondly, it can prevent or reduce thefeedback of light from the discharges to the photoconductor (this canalso be achieved by employing a separate opaque layer between thephotoconductor and the plugged slab). However, it may be desirable forthe plate to be only partially opaque so as to allow a controlled amountof optical feedback as will be explained.

The energy efficiency of the discharge can be in theorder of about Ilumen per watt for a neon-argon mixture', but maybe raised still furtherby using a gas with whiterdi'ght, or alternatively by employing awavelength conversion from ultraviolet to visible light via an efficientphosphon-ahalogous to the method employed in fluorescent tubularlamps/The phosphor may be laid on the anode or on the walls containingthe cathode glow or (in the case of positive-column glow) both glowzones.

The electrical characteristic of the photoconductor gasdischargecombination may be further improved by adding a certain amount of seriesresistance. This can be realized in the form of an additional resistivelayer, either at the cathode or at the anode side of the sandwich panel.At the cathode side it may be combined with the opaque layer mentionedabove by using a material with both the desired resistivity andabsorption.

As aforesaid, a controlled amount of feedback from the gas discharge tothe photoconductor may be beneficial in certain applications, since itcan increase the light gain of the device as well as the afterglow. Thismay be built into the panel as a permanent amount of light feedbackthrough the plugged slab or the additional opaque layer. I

Alternatively, it is possible to vary the amount of feedback in thedevice during operation as required by adjusting the absorption of asuitable material used for the plugged slab or the opaque layer. Thismay, for instance, be done by flooding the device for a short time withradiation of a suitable wavelength which adjusts the number ofabsorption centers in the material for the light of the gas discharge.

A device as described above is essentially a flat imageconverter-intensifier, with the current through each individual gasdischarge cell (and therefore its light output) determined by thecorresponding photoconductive element. With a photoconductor of thesintered CdSe type and a gas discharge with only I lumen/wattefficiency, a lumen gain to visible light of more than 10 may be readilyobtainable.

A panel of this kind can not only be employed as the flat output screenof a scanned display, but can also be used as an image converter panelin its own right. Accordingly more specific embodiments of the inventionwill now be described by way of example with reference to FIGS. 4 and 5of the accompanying drawings.

1. FLAT X-RAY IMAGE CONVERTER PANELS By using a photoconductor layerwhich absorbs a suffciently large proportion of the incident X-rayradiation it is possible to construct an X-ray converter panel such asthe ar rangement shown in FIG. 4 and such a panel may, for instance, bebuilt up in the following way.

A sheet of aluminum provides (with a glass plate WI) the entrance windowto X-rays and acts at the same time as the negative electrode E. On topof this (and possibly separated from it by a thin film ofa suitabledifferent metal to give good electrical contact) is applied a layer of,for instance, 0.5 mm. of sintered CdSe. This is covered with a thinsheet of sintered black glass paste, in which (before the sintering) alarge number of holes have been punched. These holes are partly filledup with a suitable metal, or combination of metals K-Kc to anappropriate depth to provide a good electrical contact with thephotoconductor layer underneath and a good cathode for the gas dischargeabove. This can, for instance, be done by electrolytic deposition, usingthe combined aluminium-photoconductive layer as one electrode. On top ofthe glassy sheet can be placed the cover glass carrying the anode in theform of a transparent film or (as shown) as a fine wire mesh or grid Am.The appropriate gas filling having been provided, the assembly can thenbe sealed around the edges to provide the complete panel.

In the form shown in FIG. 4, the anode elements Am are a single set ofparallel wires in contact with both the envelope or cover plate W0 and aglass base G. In principle such wires could be replaced by a grid in theform of a perforated metal plate arranged to isolate separate gaschambers for individual picture elements. However, this is not found tobe necessary and the wire arrangement of FIG. 4 is convenient in that itpermits evacuation and gas filling from the ends of the elongated spacesdefined by pairs of wires.

Other methods of construction are, of course, possible and couldincorporate wires coated with black glaze and fused together underpressure and high temperature so as to lead to an array of metal plugsin a glass plate similar to the one illustrated in FIG. 2. Such a slabmay again have the wires etched back on one side to provide the hollowcathodes, and be covered at its outer side with the photoconductor as inFIG. 3.

2. FLAT IMAGE CONVERTER PANELS If light is used as the incidentradiation, it may be necessary to employ surface photoconductivity,instead of conduction through the thickness of the photoconductivelayer. This depends on the absorption coefficient of the material andthe difbe chosen for its sensitivity to match the radiation of theapplication. This can be either ultraviolet, visible, near infrared oreven far infrared radiation. The main requirement is that the darkcurrent besufficiently low compared with the photocurrent to allow areasonable picture contrast.

3. FLAT DISPLAY PANELS FOR TELEVISION TYPE PICTURES Possibly the mostimportant application for a panel of the type described in the previoussection is for the display of sequential information. In that case itmay be activated by a flying light spot modulated with the incomingvideo signal, the decay time of the photoconductor providing therequired storage action in the actual display. Alternatively a linesequential activation can be used.

The following arrangements may be adopted:

a. Activation by a small cathode ray tube Due to the lightintensification of the panel, it is possible to use a very small andsimple C.R.T. with a low output and couple this optically via a cheapand low-aperture lens to the panel. The big advantage of the display isthat it can now provide a brighter picture than the direct viewingC.R.T. and in addition that it can have a much better performance inhigh ambient illumination, due to the black background of the glassplate with holes.

.b. ACTIVATION BY A SMALL SCANNING LIGHT BEAM Again, as a result of thelight intensification capability of the panel, it is possible to scanthe photoconductor with a low-output laser, either of the neon-heliumtype, or possibly even a solid-state laser, such as GaAs.

c. Activation by a matrix address panel As mentioned at the beginning,solid-state crossbar matrix displays are in development where thebrightness is not sufficiently high. This need not however, be a seriousproblem if such a panel is used only as the activating element for a gasdischarge intensifying panel according to the invention. The two panelscan in that case be put in direct contact with one another with thecrossbar system providing the low-intensity scanned picture which isthen intensified and made continuous' 'by the second panel.

the X-ray panel in the form of a thin continuous layer, and can Anadditional advantage of this approach is that the spectral response ofthe photoconductor is not necessarily the same as that of the eye. CdSe,for instance seems sufficiently sensitive at about 9000 A. to allowexcitation by a matrix of GaAs cells consisting of P-type strips atright angles to n-type strips. Again, it is possible for the activatingpanel to employ circuitry which allows line storage, as describedbefore, in order to increase the input brightness to the final displaypanel.

The activation panel may alternatively consist of a crossbar type ofpanel based on electroluminescent phosphors, either in thedot-sequential mode of operation-or with line storage.

As a further alternative the activating panel may be a cross bar deviceof the kind described in copending application Ser. No. 699,269, filedJan. 19, 1968.

A variety of individual cell structures will now be described in greaterdetail with reference to FIGS. 6A to 6M (referred to simply as A to M).In these FIGS. it has been found convenient to separate theglow-discharge stage of the cell from its photoconductive stage since toa large extent the design of the glow-discharge state is independentfrom that of the photoconductive stage and many combinations arepossible.

FIGS. A to G show various glow discharge arrangements and the schematicdrawings terminate at a level corresponding to the cathode emissivesurfaces K.

In each of these FIGS. the cathode surface K is located in a recess in acathode plate G (which may be of glass and may be opaque or partiallyopaque for the reasons stated above). The sidewalls of the recess may ormay not be covered by sputtered cathode material or rendereddeliberately conductive in some way. In each case the common anode isshown at A as applied to a transparent output window W0. The schematicrepresentation of the anode is intended to include both the case wherethe anode is a continuous layer which is sufficiently transparent forviewing of the glow discharge image, and also the case in which it isopaque but has regular apertures corresponding to the individual] cellsor a sufficient number of small apertures which are in a random patternor in a regular pattern unrelated to the cathode spacing.

FIG. A corresponds to FIG. 3 and does not require further comment as toits operation. The gap between the edge or rim of a recess and the anodeis shown at g (as is also done in FIGS. B to D).

In order to permit the arrangement of FIG. A to be assembled as asandwich construction, it is possible to place the output window WO,during assembly, directly on the cathode plate G, relying oninaccuracies or roughness of the two adjacent surfaces to allow freemovement of gas between individual cells and also to prevent sputteringof the cathode material from setting up conductive paths between thecathodes and the anode. Alternatively it is possible to increase theroughness of one or both of the mating surfaces artificially (forexample by sandblasting or etching) or to add to either the window plateWO or the cathode plate G a regular pattern of ridges or lugs to act asspacers between the two plates. If such regular spacers are applied tothe window W0 this may introduce a requirement for registration betweenthe spacers and the cathode recesses. This need for registration can beavoided by forming the regular spacers on the cathode plate instead,e.g. as shown at S in FIG. B. This last alternative is also preferablein that it permits the common anode to be formed as a continuous layeron the plate W0.

FIG. C shows a further alternative arrangement in which the anode ofeach cell is no longer formed as a surface on the output window oppositethe cathode recess. Instead the anode is provided as a metal grid ormesh AM e.g. in the manner described with reference to FIG. 4 so thatthe anode surfaces of each cell surround or border on the cathode recessinstead of being located opposite the recess. Such a grid or mesh ispreferably a grid in which each mesh surrounds one cathode recesswithout, however, forming gastight seals. This calls for a high degreeof registration between the meshes of the grid and the cathode recesses(although it has the advantage of providing a sandwich typeconstruction). The need for registration can be obviated by replacingthe grid Am by a network of ridges being rendered conductive so as toprovide the anode surfaces. An example of this is shown in FIG. D wherethe conductive anode surfaces are indicated at Ad.

FIG. E shows an alternative construction which is designed to produce apositive column glow in addition to a cathode glow. This is done bymaking the cathode recess much deeper so as to cause production of apositive column glow in addition to the cathode glow. If there is asingle continuous recess as shown, there is some need of a gap (asshown) between the anode A and the output window and the cathode plateGC as in FIG. A, or alternatively a sufficient degree of irregularroughness at one or both of the mating surfaces or a regular anode orspacer pattern as described with reference to FIG. B to D (such apattern is indicated generically at AS).

As is well known in the art, the provision of a discharge space havingthis type of geometry, combined with known methods of determining thegas pressure, can give rise to emission of light from a positive columnin the region between the cathode glow and the anode.

To avoid the possibility of sputtering gradually affecting the length ofthe positive column in different degrees for different cells, FIG. Fshows an alternative construction in which the deeper discharge spacefor the positive column glow is provided opposite each cathode recesswithin a separate matrix structure C, spaced from the cathode plate G bya gap.

In this arrangement the gap or spacers of FIGS. A to D are no longerneeded at the anode. As for the gap between cathode plate G and matrixC, it may contain regular or irregular spacer elements formed on eithersurface as indicated at S.

The arrangement of FIG. F has an anode layer A which may be a continuoustransparent layer or a pattern of opaque conductive material asexplained with reference to FIG. A. If it is a continuous layer, thenthe need for registration between the anode and the matrix C is avoidedalthough such registration isstill required in this case between thematrix and the cathode recesses.

An alternative arrangement for the anode of FIG. E or FIG. F, whichstill avoids the need for anode registration, is shown in FIG. G inwhich a conductive layer is applied to the whole of the output face ofthe matrix C and extends inwardly to a predetermined extent into each ofthe discharge spaces. In this case the conductive anode layer A can beopaque and it can be obtained by an evaporation process in which theevaporation is directed at a suitable angle to the face of the matrix soas to ensure the desired depth of penetration into the discharge spaces.A number of ossible variants of the photoconductor stage will now bedescribed with reference to FIGS. H to M. Any of the arrangements ofFIGS. H to M can be combined with any of the discharge stagearrangements of FIGS. A to G.

FIG. II shows an arrangement in which all the cathode surfaces K are incontact with a continuous layer P of photoconductive material. Thecontact between each cathode surface and said layer may either be director it may be indirectly obtained through a separate coating or plug ofcontact" metal such as the layer Kc shown in FIG. 4.

An input window WI is shown adjacent to the layer P but said window isnot essential to the structure which may, in certain applications bepreceded by an imaging stage of some kind. On the layer P there is showna negative supply electrode E. which may either be semitransparent ormay be formed as a pattern of opaque conductive material havingapertures such as to pass input radiation Ri from an object to the layerP. Such apertures may be in regular pattern in registration with thecathode surfaces K although random patterns can be used if suflicientlyfine.

The simple arrangement of FIG. 'I-I corresponds effectively to thearrangements used in FIGS. 3 and 4. In each case the photoconductiveaction occurs in the form of photocurrents substantially in thedirection of the thickness of the layer P as indicated by the arrow X.

A variant of the arrangement of FIG. H is shown in FIG. I where the onlychange is the subdivision of the photoconductive material into separateplugs each of which is in series with one of the discharge cells. Inthis case the photoconduction still occurs in the direction of the arrowX.

The arrangement of FIG. .I corresponds essentially to that used in FIG.5. Here the photoconductive material is subdivided into separate islandseach of which surrounds and is in contact with a cathode plug Kp. Eachphotoconductive island is surrounded by conductive material forming anegative supply electrode Em of grid or mesh form. Although this grid isshown as having the same thickness as the photoconductive islands sothat it effects the subdivision of the photoconductive material, this isnot essential and the grid may in fact be either exposed or, conversely,buried inside the photoconductive material. In particular the problem ofregistration between this grid and the cathode plugs can be overcome byproviding the mesh on the cathode itself. For example, it may beproduced as a printed pattern of metal on the cathode plate as indicatedin FIG. K (or recessed therein) or it may be provided as a layer on agridlike network of ridges formed on the cathode plate as shown in FIG.L.

It is not necessary for the cathode plugs Kp to be exposed and, indeed,they may be made flush with the cathode plate G. If the negative supplyelectrode E" is'also flush with the plate G, then the photoconductivematerial P may be formed as a continuous layer having constant thicknessas shown in FIG.

In any event, the photoconductive action of the arrangements of FIGS. Jto L is in the form of photocurrents which flow mainly in directionssubstantially parallel to the plane of the device as indicated by thearrows X.

A set of practical dimensions and values is given below by way ofillustration:

Reverting to the question of providing electrical resistance in serieswith the cells, a continuous resistive layer may be used, as aforesaid,and may be located between the photoconductor and the cathodes or (ifused) cathode plugs. Such layer may also provide a degree of opacity ortranslucence which may be required for optical purposes as explainedmore fully below. As an alternative to a continuous layer, individualcathode plugs (such as the plugs Kp of FIGS.'6J to 6M) may be made ofresistive material to act as individual resistors.

Reverting now in greater detail to the questions of optical feedback andcontrast in high ambient illumination, various specific cases can arise,some of which concern the further possibility of displaying additionalinformation or an additional image by back-projection (the latter termis used to denote systems in which the glow-discharge image hassuperimposed on it a second image which is in light of a wavelengthwhich substantially does not effect the photoconductor and thus leavesthe discharge pattern undisturbed). The following are relevant cases:

I. The material between the cathode recesses is nonreflective and opaquefor both glow light and ambient light (this prevents optical feedbackand improves contrast by reducing reflection of ambient light).

II. Said material is translucent but not for glow light (this preventsfeedback but permits back-projection in a color differing from that ofthe glow discharges).

III. Said material is translucent to glow light and the photoconductoris sensitive to glow light (this permits a controlled degree of feedbackdependent on the degree of translucence.)

IV. Said material is translucent for glow light but the photocbnductoris not sensitive thereto (thus feedback will not occur).

V. The translucence is arranged so as to pass light of a thirdwavelength different from both that of the glow light andthat of thelight of the intended input image radiation, the photoeonductor beinginsensitive to light of said third wavelength (this permitsback-projection in a color different from that of the glow light).

In the above description the term light is used to cover also invisibletypes of light, for example ultraviolet and infrared.

lclaim:

1. An electrical display panel device comprising a twodimensional arrayof glow-discharge cells, said array further comprising an insulatingbase plate having a plurality of apertures therein, a conductive pinhaving an emissive surface within each of said apertures, the emissivesurface of said pin being spaced from one surface of said plate anddefining therewith a recess in the plate the walls of which are coveredwith said emissive material and constituting a cathode cell of saidarray, a common anode for all the cells of the array spaced from theedges of each of the cells and through which the cells can be viewed, aninert gas filling said recesses at a pressure at which with a givenpotential between the anode and at least one of the cathodes there is anegative glow discharge which fills the recess and a common negativesupply electrode for all the cells which electrode is connected to eachcathode through an element of photoconductive material and allowspassage of input radiation to each of said elements.

2. A display device as claimed in claim 1 in which the negativeelectrode is a single continuous photoconductive layer arranged to becommon to all the cells.

3. A display device as claimed in claim 1 in which the negativeelectrode comprises a separate photoconductive element for each cell.

4. A display device as claimed in claim 1 in which the negativeelectrode is positioned so that the photoconductive action is mainly inthe direction of thickness of the display device.

5. A display device as claimed in claim 3 in which the elements arepositioned so that the photoconductive action of each element is mainlyin directions substantially parallel to the image of the display device.

6. A display device as claimed in claim 1 wherein the gas filling iscontained between an output window plate and the plate in which thecathode recesses are formed.

7. A display device as claimed in claim 6 wherein the plate between thecathode recesses is opaque.

8. A display device as claimed in claim 6 wherein the plate between thecathode recesses is translucent.

9. A display device as claimed in claim 1 including an electricalresistance in series with each of the cells.

V UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 5Dated January 5 1971 Pieter Schagen Inventor(s) It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3, line 65 after "recess" insert and separated therefrom by a gapto minimize sputtering on the walls Signed and sealed this 13th day ofApril 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, Attesting OfficerCommissioner of Pater

1. An electrical display panel device comprising a twodimensional arrayof glow-discharge cells, said array further comprising an insulatingbase plate having a plurality of apertures therein, a conductive pinhaving an emissive surface within each of said apertures, the emissivesurface of said pin being spaced from one surface of said plate anddefining therewith a recess in the plate the walls of which are coveredwith said emissive material and constituting a cathode cell of saidarray, a common anode for all the cells of the array spaced from theedges of each of the cells and through which the cells can be viewed, aninert gas filling said recesses at a pressure at which with a givenpotential between the anode and at least one of the cathodes there is anegative glow dischargE which fills the recess and a common negativesupply electrode for all the cells which electrode is connected to eachcathode through an element of photoconductive material and allowspassage of input radiation to each of said elements.
 2. A display deviceas claimed in claim 1 in which the negative electrode is a singlecontinuous photoconductive layer arranged to be common to all the cells.3. A display device as claimed in claim 1 in which the negativeelectrode comprises a separate photoconductive element for each cell. 4.A display device as claimed in claim 1 in which the negative electrodeis positioned so that the photoconductive action is mainly in thedirection of thickness of the display device.
 5. A display device asclaimed in claim 3 in which the elements are positioned so that thephotoconductive action of each element is mainly in directionssubstantially parallel to the image of the display device.
 6. A displaydevice as claimed in claim 1 wherein the gas filling is containedbetween an output window plate and the plate in which the cathoderecesses are formed.
 7. A display device as claimed in claim 6 whereinthe plate between the cathode recesses is opaque.
 8. A display device asclaimed in claim 6 wherein the plate between the cathode recesses istranslucent.
 9. A display device as claimed in claim 1 including anelectrical resistance in series with each of the cells.